The electrical conductivity is most sensitive to the presence of fluids in the host rock and may change over orders of magnitude depending on water content, presence of partial melt and connectivity. Electromagnetic (EM) experiments, as f.ex. Magnetotellurics (MT), measuring the electrical resistivity distribution are therefore a method of choice for imaging water and partial melt distributions.

While Germany has a great expertise in land EM, we are building up, within the framework of the SFB, the first German marine EM group. The work of the group thus encompasses design and construction of marine EM instrument, data acquisition, data processing as well as data interpretation modules.

Marine EM Instruments

We constructed highly sensitive marine MT Stations (figure 1) within the SFB574 together with the KUM Technologies, Kiel and Magson GmbH, Berlin. The stations are free falling and measure electric and magnetic field variations and tilt variations on the seafloor for a

duration of up to 6 months. From the time series we may deduce magnetotelluric responses in a period range from 0.1s to several days which may give information of resistivity structure from a depth of several kilometres down to hundreds of kilometres.

Data acquisition

The MT stations were used in 2007-2008 to conduct a long-period offshore magnetotelluric investigation in north western Costa Rica, where the Cocos Plate subducts beneath the Caribbean plate. Eleven marine magnetotelluric Stations were deployed on the 200 km long marine profile for several months, which is an extension of a land profile carried out by the Free University of Berlin over length of 160 km with further 18 stations.

The MT data set revealed a number of interesting features imaging the Hydration and Dehydration of the Costa Rican Subduction Zone which are discussed below.

Data processing

Data processing of marine MT data is very challenging due to the fact that tidal current and waves may shake the stations intermittently (especially for shallow water and continental shelf data). We are therefore working on developing new MT data processing techniques that incorporate tilt measurements and allows us to work in the time as well as spectral domain.

For the latter we are using Hilbert Huang transforms and Empirical Mode Decomposition which allow a representation of the electric and magnetic data in time and frequency and furthermore a magnetotelluric response function determination (figure 2).

Data interpretation:

The Amphibious MT profile across the Costa Rican subduction zone and pseudosections of the TE and TM mode MT responses are shown in figure 3. These responses depict the subduction zone highlighted through electrical current along and across strike respectively and are to be understood as a strongly smoothed indication of resistivity variations with increasing depth (corresponding to increasing periods). While the amphibious data is consistent within each mode, the difference between the two modes show that the underlying resistivity distribution is highly two dimensional. We are currently inverting the amphibious responses to an electrical resistivity model.

Figure 1: IFM-Geomar marine MT station

Figure 2: Right side: Hilbert spectra (frequency versus time) of electric and magnetic field data measured at the seafloor. Left side: Derivation of MT response function through linear fit of electric and magnetic field amplitudes pairs within a frequency band at different times.

Figure 3: Upper panel: Layout of amphibious MT profile. Bottom panels: MT response (apparent resistivity) along profile as function of period for the TE mode and TM mode (corresponding to currents flowing along and perpendicular to strike).

Hoelz , S., M. Jegen, 2009. Development of a CSEM system for the electromagnetic investigation of the North Alex Mud Volcano, European Geosciences Union General Assembly, Vienna, Austria, 19th to 24th April.